This invention relates to a recording medium particularly suitable to use with ink jet printers and to a method for the treatment of images prepared therewith. It particularly relates to images produced using so-called aqueous inks, that is to say inks in which water comprises the major component of the liquid phase. There is increasing interest in the use of aqueous inks for environmental and safety reasons.
Printing media suitable for use with ink jet printers are well known. Commonly these employ at least one ink receiving layer coated on a suitable substrate. The purpose of the receiving layer is to take up the ink rapidly and improve image quality. One problem with images produced using aqueous inks is that they can be insufficiently robust to handling, and that the image or the receiving layer on which it is printed is sensitive to rubbing and scratching. In addition the printed image frequently becomes more sensitive to handling and damage under damp conditions, and can sometimes be washed completely away.
Several methods of overcoming the poor robustness of images produced using aqueous inks are known. For instance various additional coatings and treatments for ink receiving layers have been proposed, such as lacquers or varnishes which have to be applied after printing the image, thus requiring additional equipment. For instance our British Patent Application 2337482 A provides a method for increasing the rub resistance of an image by coating or over-printing the image with an aqueous solution of a styrene acrylate polymer.
Another method of improving the robustness of printed images is to laminate or encapsulate them, and this is particularly common when they are intended for external display. By lamination is meant the combination of a printed image with a transparent overlay, this combination usually being accomplished with an adhesive activated by heat, pressure, or both. The overlay acts as a physical protection for the image and completely seals it from ingress of water. By encapsulation is meant the combination of a printed image layer between two laminating sheets, that on the image surface being transparent, the combination being accomplished with an adhesive activated by heat, pressure, or both. Encapsulation is most effective if the laminating sheets extend beyond the printed image and are bonded to each other at the extremities, thus preventing ingress of water through exposed edges of the image.
However lamination and encapsulation are both expensive because additional materials are required together with additional handling and equipment, and there is considerable interest in finding a cheaper and simpler method of increasing the robustness of images produced using aqueous inks.
As an alternative to lamination or overprinting, Japanese Patent Applications 59/222381, 07/237348, 08/02090, and 09/1104164 and European Patent Applications 0 858 905 and 0 858 906 disclose a heat seal method of protecting an ink jet image wherein the receiving system comprises two layers coated on a suitable base. The lower layer is an ink receiving layer which is absorbent to the ink, whereas the upper layer comprises a film forming polymer in a binder. After printing the upper layer may be sealed by heating to form a robust barrier to protect the image in the lower layer. This is similar to laminating the image, but does not require the additional expensive lamination sheet.
However this heat seal method needs to achieve high temperatures to seal the image (up to 170° C. being given in the Examples of EP 0 858 906 A) and also requires a relatively complicated and expensive receiving sheet. There is thus still a need for an imaging material and method which will provide images resistant to washing and handling when printed using aqueous inks without lamination. We have found a material and method which achieves these objectives.